In order to obtain the dimensions of the ring, the first step is to make measurements of the existing dynamo ring. It helps here to use a caliper to measure the radius of the ring. Measurements should be taken at the top and at the bottom as the ring has a slight taper.

The ring was modeling in Blender. Other 3D modeling programs can be used here such as OpenSCAD or Fusion360. The first step was to create a cylinder with the same upper and lower dimensions measured in step 1 and thus follows the shape of the existing dynamo wheel, for now we call this shape DynoCylinder. Following a second cylinder is generated (Ring) which fits over DynoCylinder. The Ring needs grooves in order to provide some grip on the bicycle tire. These grooves can be generated by subdividing the Ring and extruding the required faces. To minimize repetitions, the array modifier in Blender can be used to repeat shapes.

Using the boolean blenders modifier, the DynoCylinder is subtracted from Ring, leaving only the outside ring shape which will be printed.

Both of these shapes are visible in the attached Blend file and can be modified to fit other dynamo configurations. The final STL is also attached.

The model was designed to be printed without supports. The TPU requires slightly lower print speeds and a heated bed for good bed adhesion. I used 40mm/s and 70 degrees.

A bit of superglue does a great job of adhering the TPU to the old dynamo wheel. Just clean off any dirt and grime, place a couple dots of glue in the new Ring and slide onto the old dynamo wheel.